Life cycle of an extension

A PHP extension goes through several phases during its lifetime. All of
these phases are opportunities for the developer to perform various
initialization, termination, or informational functions. The Zend API allows
for hooks into five separate phases of an extension's existence, apart from
calls by PHP functions.

Loading, unloading, and requests

As the Zend engine runs, it processes one or more "requests" from its
client. In the traditional CGI implementation, this corresponds to one
execution of a process. However, many other implementations, most notably
the Apache module, can map many requests onto a single PHP process. A PHP
extension may thus see many requests in its lifetime.

Overview

In the Zend API, a module is loaded into memory only once when the
associated PHP process starts up. Each module receives a call to the
"module initialization" function specified in its
zend_module structure as it is loaded.

Whenever the associated PHP process starts to handle a request from its
client - i.e. whenever the PHP interpreter is told to start working - each
module receives a call to the "request initialization" function specified
in its zend_module structure.

Whenever the associated PHP process is done handling a request, each
module receives a call to the "request termination" function specified in
its zend_module structure.

A given module is unloaded from memory when its associated PHP process is
shut down in an orderly manner. The module receives a call to the "module
termination" function specified in its
zend_module structure at this time.

What to do, and when to do it

There are many tasks that might be performed at any of these four points.
This table details where many common initialization and termination tasks
belong.

Aside from globals initialization and certain rarely-used callbacks, there
is one more part of a module's lifecycle to examine: A call to
phpinfo(). The output a user sees from this call, whether
text or HTML or anything else, is generated by each individual extension
that is loaded into the PHP interpreter at the time the call is made.

To provide for format-neutral output, the header
"ext/standard/info.h" provides an array of functions to produce
standardized display elements. Specifically, several functions which create
the familiar tables exist: